You might consider getting ALL your staff together and making a general
announcement that items are going missing, and that everyone should
take more care to be sure their belongings are safe. That not only gets
the idea across to the people who aren't stealing that they should be
cautious, it also tells whoever is stealing that you are aware and
watching.

If someone asks if there is someone specific you have in mind, honestly
say you're not in a position to make accusations, but you are able to
tell everyone to be careful.